Climate Fact: What Controls Atlantic Tropical Cyclone Development?
In Brief: Sea surface temperatures in the Main Development Region, conditions in the tropical Pacific Ocean and the North Atlantic Oscillation are all important factors for determining the likelihood of tropical cyclone formation in the Atlantic Ocean.
Predicting the formation of tropical cyclones in the North Atlantic basin is challenging but critical considering the severe socio-economic impacts landfalling hurricanes have on the populated regions of the East, Gulf and Caribbean Coast regions. Historical analysis and model experiments suggest that the following factors are of major importance to Atlantic tropical cyclone frequency:
- Sea Surface Temperatures in the Main Development Region (MDR SSTs): The majority of Atlantic tropical cyclones are born in the Main Development Region (MDR) between 20 and 60 degrees West and 6 and 18 degrees North – from off the coast of Liberia in West Africa northwest to around Antigua in the West Indies. Warm sea surface temperatures, particularly if the heat in these waters generates sufficient convection to create a layer of cumulus clouds, helps small cyclonic disturbances in the easterly flow of tropical winds to grow. Waters in the MDR generally need to be above 80 degrees Fahrenheit for tropical storm formation. Over the 20th century, the average temperature of these waters increased by over one degree Fahrenheit to the warmest levels on record.
- The El Niño-Southern Oscillation (ENSO): Conditions in the tropical Pacific Ocean affect tropical cyclone formation in the Atlantic by affecting vertical wind shear. Vertical wind shear describes how much wind speed and direction changes as you move higher in altitude, with more uniform conditions (less vertical wind shear) being more favorable to tropical cyclone formation. Traditional El Niño conditions, with the warmest waters concentrated in the eastern tropical Pacific, increase the temperature contrast between the tropical and subtropical regions, which works to increase the amount of vertical wind shear over the Atlantic MDR. In recent decades, however, another type of El Niño with the warmest waters concentrated in the central tropical Pacific has become more common. These types of El Niño events do not suppress vertical wind shear over the Atlantic. La Niña conditions, or cool waters in the tropical Pacific, also do not suppress vertical wind shear, making tropical cyclones more common during La Niña years.
- The North Atlantic Oscillation (NAO): The North Atlantic Oscillation (NAO) describes how the strength of the westerly winds blowing over the North Atlantic varies over time. While the mechanism behind the NAO’s influence on tropical cyclone formation remains unclear, statistical correlations between the NAO and Atlantic tropical cyclone counts exist, suggesting a relationship.
While these variables help scientists predict when tropical cyclones are more or less likely to occur, it is important to remember that just because tropical cyclones are unlikely to develop does not mean that they will not.
Seasons: Summer, Fall
Sources: Kozar, ME et al. “Stratified statistical models of North Atlantic basin-wide and regional tropical cyclone counts.” Journal of Geophysical Research: Atmospheres 117 (2012): D18103 and Larson, S et al. “Impacts of non-canonical El Niño patterns on Atlantic hurricane activity.” Geophysical Research Letters 39 (2012): L14706 and Stan, C. “Is cumulus convection the concertmaster of tropical cyclone activity in the Atlantic?” Geophysical Research Letters 39 (2012): L19716.